10 not at all meaning the immoral of this world, or the greedy and robbers, or idolaters, since then you would need to go out of the world. 11 But rather I wrote to you not to associate with any one who bears the name of brother if he is guilty of immorality or greed, or is an idolater, reviler, drunkard, or robber--not even to eat with such a one. 12 For what have I to do with judging outsiders? Is it not those inside the church whom you are to judge?
Matthew Henry's Commentary on 1 Corinthians 5:10-12
Commentary on 1 Corinthians 5:9-13
(Read 1 Corinthians 5:9-13)
Christians are to avoid familiar converse with all who disgrace the Christian name. Such are only fit companions for their brethren in sin, and to such company they should be left, whenever it is possible to do so. Alas, that there are many called Christians, whose conversation is more dangerous than that of heathens!